1. Field of the Invention
The present invention relates to a photovoltaic cell including a substrate glass made of aluminosilicate glass having high thermal resistance, low processing temperature, and high crystallization resistance.
2. Description of the Related Art
Soda-lime glass, also referred to as alkaline-earth-silicate glass, is among the oldest known glass types and is the most frequently used type of glass with respect to quantity (“normal glass”). It is used in many fields, for example as plate glass, such as mirror or window glass, as packaging glass, for example, for bottles, food packages, and drinking glasses, or in automobile construction, as composite safety glass. Known soda-lime glasses have the disadvantage that they only have a relatively low thermal resistance, for example, having a transformation temperature (“Tg”) in the range of approximately 490 to 530° C. The application of soda-lime glasses is significantly restricted in this way.
The properties of glass and the products produced therefrom must be specially adapted for many applications. Therefore, a need exists to vary and modify the known glass compositions in such a manner that the properties of the glasses are suitable for special applications. However, it is always problematic in this context that the reduction or increase of the proportion of one component can trigger a plurality of effects which act differently on the glass properties. The procedures and effects in the event of replacement or modification of multiple components in a glass composition are still more complex, the individual glass components mutually influencing one another, so that simple relationships are typically not predictable. It is therefore relatively difficult to provide custom-tailored glass compositions for special applications.
There are numerous publications in the prior art which are concerned with glasses according to the species. For example, JP 07-101748 A describes low-aluminum, alkaline glasses for plasma display panels, the glass composition being synthesized from 0.3-2.5 weight percent (wt.-%) Li2O, 7.0-12 wt.-% Na2O, 1.5-4.5 wt.-% K2O, 0-5.0 wt.-% MgO, 6.0-9.0 wt.-% CaO, 0-5.0 wt.-% SrO, 3.5-15.0 wt.-% BaO, 2.0-4.5 wt.-% Al2O3, 57.0-68.0 wt.-% SiO2, 0-5.0 wt.-% ZrO2, and 0-0.5 wt.-% CeO2, the sum of Li2O+Na2O+K2O being 9.0-16.0 wt.-%.
U.S. Pat. No. 5,858,897 describes a glass composition for a substrate, which is suitable in particular for a flat display, preferably a plasma display panel (PDP). The glass composition essentially comprises: 59-72 wt.-% SiO2, 1-15 wt.-% Al2O3, 0.5-9 wt.-% MgO, 0.5-11 wt.-% CaO, 0-6 wt.-% SrO, 0-5 wt.-% BaO, 4-19 wt.-% MgO+CaO+SrO+BaO, 0-9 wt.-% Na2O, 4-21 wt.-% K2O, 10-22 wt.-% Na2O+K2O, and 0.5-10.5 wt.-% ZrO2, the difference between the SiO2 content and the Al2O3 content being 50 to 71 wt.-% and the relative density being less than 2.6.
Furthermore, EP 0 769 481 A1 discloses a glass composition for a substrate, in particular for plasma displays, the glass composition comprising: 52-62 wt.-% SiO2, 5-12 wt.-% Al2O3, 0-4 wt.-% MgO, 3-5.5 wt.-% CaO, 6-9 wt.-% SrO, 0-13 wt.-% BaO, 17-27 wt.-% MgO+CaO+SrO+BaO, 7-14 wt.-% Li2O+Na2O+K2O, 0.2-6 wt.-% ZrO2, and 0-0.6 wt.-% SO3. Such a high SrO proportion in the glass composition has great disadvantages, however. SrO is a relatively costly material, so that the production of the glass becomes significantly more expensive. The advantages asserted in EP 0 769 481 A1, that the transformation temperature is to significantly increase and the thermal coefficient of expansion is to rise, could not be reproduced according to the present invention. Rather, an elevated SrO content does not display any positive influence on the properties and effects for the areas of application according to the present invention; SrO is therefore not provided in the high quantities described here according to the present invention.
Furthermore, EP 0 879 800 A1 describes solarization-stable aluminosilicate glasses, which are suitable for use in display technology, in particular for plasma display panels, and have the following composition:    SiO2 45-68 wt.-%;    Al2O3 >5-18 wt.-%;    Na2O 0-5 wt.-%;    K2O >9-15 wt.-%;    Na2O+K2O ≧10 wt.-%;    CaO 0-10 wt.-%;    SrO 0.5-18 wt.-%;    BaO 0-10 wt.-%;    CaO+SrO+BaO 8-<17 wt.-%;    ZrO2 1-6 wt.-%; and    TiO2 0.2-5 wt.-%.
U.S. Pat. No. 5,958,812 A relates to thermally stable glass compositions, which comprise:    SiO2 45-68 wt.-%;    Al2O3 0-20 wt.-%;    ZrO2 0-20 wt.-%;    B2O3 0-10 wt.-%;    Na2O 2-12 wt.-%;    K2O 3.5-9 wt.-%;    CaO 1-13 wt.-%;    MgO 0-8 wt.-%;    the sum of SiO2+Al2O3+ZrO2≦70 wt.-%, the sum of Al2O3+ZrO2≧2 wt.-%, and the sum of Na2O+K2O≧8 wt.-%, and if desired, the oxides BaO and/or SrO being provided in quantities such that: 11 wt.-%≦MgO+CaO−BaO+SrO>30 wt.-%. The glass compositions are used for the production of fire-resistant window panes, in particular plasma shields, electroluminescence shields, and cold cathode shields (field emission displays).
Furthermore, US 2005/0003136 A1 discloses a glass composition which obtains a higher strength through chemical treatment. The glass composition comprises:    SiO2 59-68 wt.-%;    Al2O3 9.5-15 wt.-%;    Li2O 0-1 wt.-%;    Na2O 3-18 wt.-%;    K2O 0-3.5 wt.-%;    MgO 0-15 wt.-%;    CaO 0-15 wt.-%;    SrO 0-4.5 wt.-%;    BaO 0-1 wt.-%;    TiO2 0-2 wt.-%; and    ZrO2 1-10 wt.-%.    The glass composition is used, for example, as a glass substrate for a magnetic recording medium.
Furthermore, aluminosilicate glasses having a high thermal resistance and low processing temperature are known from WO 2011/035889 A1 (DE 10 2009 042 796.1), filed on Sep. 25, 2009. Experiments have shown that the glasses do have the required physical properties, but in specific cases do not display satisfactory results with respect to the crystallization resistance.
In particular with respect to renewable or regenerating energy, i.e., energy from sources which either renew themselves in a short time or whose usage does not contribute to the exhaustion of the source, novel and improved possible uses of glasses are increasingly significant. The limited resources of fossil energy carriers, the need for environmental and climate protection, and the efforts for lower dependence on energy exporters or overall for a more sustainable energy provision require targeted refinements of the existing technologies. For these available sustainable energy resources, such as solar radiation (solar energy), very special requirements on the glasses to be used must additionally be considered.
A demand therefore exists to provide glasses which do not display the disadvantages of the prior art, are modified with respect to their properties, and may be used in particular in the field of converting solar energy into electrical current (photovoltaics).
Accordingly, what is needed in the art is an alternative to soda-lime glasses which avoids the disadvantages of the prior art, has a higher thermal carrying capacity (Tg) in comparison to soda-lime glasses, but nonetheless has the lowest possible processing temperatures (VA), and does not tend toward crystallization and is used in photovoltaics.